Heretofore, various types of rich-lean combustion burners have been proposed, which are characterized in that for reduction in NOx, a lean-side mixture whose air ratio (the ratio of the amount of air to the amount of fuel) is in excess of 1.0 is burned at lean-side flame holes while for the stabilization of combustion flames, rich-side flame holes, at which a rich-side mixture whose air ratio falls below 1.0 is burned, are arranged adjacent to the lean-side flame holes. As such a rich-lean combustion burner, there has been proposed a rich-lean combustion burner which is flat in shape as a whole. More specifically, this rich-lean combustion burner is formed by joining and welding together various types of formation members provided by stamping (press-forming) of thin plate materials into predetermined shapes. For example, Patent Literature Publication 1 discloses such a rich-lean combustion burner that a row of lean-side flame holes is formed by partition in the middle relative to the width direction and two rows of rich-side flame holes are formed respectively on either side of the lean-side flame hole row, whereby the rich-side flame hole rows on both sides are arrayed in opposing relation to each other across the lean-side flame hole row positioned in the middle of the burner. In addition, Patent Literature Publication 2 also discloses such a rich-lean combustion burner that two rows of rich-side flame holes of secondary burners on both sides are arranged in opposing relation to each other across a row of lean-side flame holes of a main burner. As an assembly method for assembling such a rich-lean combustion burner, there has been proposed an assembly method, in accordance with which assembly method there are prepared a lean-side flame hole formation member formed by placing a plurality of metallic sheets one upon the other so as to define a plurality of slit-like clearance gaps therebetween, and a secondary burner formed by partially connecting metallic plates on both sides by means of bridge formation at their upper ends. And the secondary burner is placed from above the lean-side flame hole formation member so as to assemble a rich-lean combustion burner. Furthermore, Patent Literature Publication 3 proposes, as a method of forming a lean-side flame hole formation member of the type described above, a formation method. In accordance with this formation method, a single metallic plate is subjected to stamping (press forming) for forming convexo-concave portions in a flame hole part. Then, the metallic plate thus prepared is folded along given fold lines into such a state that a plurality of strip-like metallic plates are placed one upon the other. These plural strip-like metallic plates are connected by joining of flat surfaces at both longitudinal ends thereof. In addition, their middle portions relative to the width direction are spaced apart a predetermined distance, thereby forming a lean-side flame hole formation member in which a great number of slit-like lean-side flame holes are formed by partition between the middle portions. And, in the middle position relative to the width direction, the lean-side flame hole formation member is fitted into a space to which a lean-side mixture is supplied, whereby the lean-side flame hole formation member is assembled, with their both ends nipped in between the ends of the rich-side flame hole formation member.